1. Field of the Invention
This invention relates to an optical filter for passing light of a specific wavelength, specifically an optical fiber, which in use in an optical transmission path of an optical fiber, has good coupling property with the optical fiber for the optical transmission.
2. Related Background Art
In optical communication generally a 1.3 .mu.m-band light and a 1.55 .mu.m-band light are used. When both wavelength ranges are transmitted, the 1.3 .mu.m-band light or the 1.5 .mu.m-band light have to be selected on the side of an optical receiver.
A dielectric multi-layer film filter among a light transmitting optical part has been conventionally used as an optical part having wavelength selectivity (the function of passing a specific wavelength and reflecting the other wavelengths). Such dielectric multi-layer film filter comprises a multi-layer of dielectrics of different refractive indexes to cause reflected light on the interfaces between the respective thin films to interfere with one another, whereby a specific wavelength can be selected.
Specifically, in using the above-described conventional optical filter in a communication system using optical fibers as an optical transmission path, in view of the coupling property (low excess insertion loss) of the optical filter, the optical fibers are micro-machined (as described in "Development of Fiber optic Passive Devices", Fukuma et al., Sumitomo Denki, March, 1990, No. 136, pp 60-67) to receive the multi-layer film filter between the optical fibers.
FIGS. 1 and 2 show the steps of fabricating the above-described optical filter. Following these steps, first, a part of the coating of an optical fiber is longitudinally removed, and the optical fiber 2 is fixed to a substrate 1 by means of an adhesive 3, with V-shaped grooves beforehand formed therein (FIG. 1).
Then, a groove 4 is cut in the part of the optical fiber 2 with the coating removed and in the substrate 1 at a set angle .theta. to the axis X of the optical fiber 2. Next, a dielectric multi-layer film filter 5 comprising dielectrics of different refractive indexes laid alternately one on another on a thin film substrate glass is inserted into the groove 4, and then filter 5 is fixed by an adhesive (FIG. 2).
The insertion of the multi-layer film filter 5 at the set angle .theta. to the axis X of the optical fiber 2 is for preventing wavelengths which have not been admitted through the dielectric multi-layer film filter 5 from returning to the side of incidence of light.
Thus, the conventional optical filter is fabricated by following many steps of machining the optical fiber, fabricating and machining the dielectric multi-layer film filter, inserting and securing the filter in the optical fiber path, and other steps.